This invention is related to apparatus for heat treating metal workpieces in a bed of hot sand, and more particularly to apparatus having means for passing a gas upwardly through the sand to fluidize it during the heat treat process.
Metal workpieces are heat treated in a fluidized bed of heated, finely divided refractory particles, such as sand. A gas or air mixture is passed upwardly through the sand to fluidize the bed. Examples of such a process are disclosed in U.S. Pat. No. 3,197,346, which issued to J. C. Munday, July 27, 1965; U.S. Pat. No. 4,379,725, which issued to Kemp, Apr. 12, 1983; U.S. Pat. No. 4.340,433, which issued to Harding, July 20, 1982; and U.S. Pat. No. 3,298,793, which issued to Mallison et. al., Jan. 17, 1967.
Although fiberglass has been suggested as a means for diffusing the gas, fiberglass tends to break down and fuse at temperatures above 1000.degree. F. Thus, it is unsuited for applications in which the temperature reaches 2500.degree. F.
At temperatures above 500.degree. C. natural gas cracks and soot and carbon tend to foul and plug up conventional metallic based diffusers.
Ceramic tile has also been suggested as a diffusing material. However, at changing temperatures, ceramic tile also cracks, due to thermal or mechanical stress. The cracked tile is ineffective because it changes the gas flow passing through the tile. Ceramic tile is difficult to attach to metal support systems because of different expansion characteristics leading to the appearance of a cracked tile.